A pressure switch is a mechanical device which converts a pressure change of a liquid or gas into an electrical function. The pressure change might be measured as pressure, vacuum, or differential between two pressure inputs. In every case, the pressure switch will employ a diaphragm, a piston, a signal transducer, or other pressure-responsive sensor which is coupled to the mechanical means of actuating a switch. Pressure switches fulfill a variety of monitoring and control applications, and they are employed in virtually every industry, from appliances to automobiles to computers. They are often used in pneumatic systems, such as air compressor pressure switches for furnaces or HVAC systems, as well as water pressure switches or oil pressure switches. Pressure switches are common components of high-efficiency heating systems as well as high-efficiency water heaters. Different manufactures make differing types of pressure switches, and each type is set according to the manufacturer's specifications.
Pressure switches activate electromechanical or solid-state switches upon reaching a specific pressure level. For example, “normally open” pressure switches are used to keep the system from operating should the pressure not be high enough or exceed the safety limit. For example, should a flue become partially plugged, the pressure in the exhaust will build up presenting a dangerous condition. Flue gases containing carbon monoxide will spill into the living space. The flames will become unstable and “float” or “spill” out of the heat exchanger creating a fire hazard. Under these conditions, the normally-open switch will not close and the furnace will not be able to run. As this example illustrates, if the pressure in a system becomes either too high or too low, depending on whether the switch is a positive pressure switch that measures positive pressures, or a negative pressure switch that measures negative (vacuum) pressures, the pressure-responsive sensor (e.g. a diaphragm within the switch) will be affected to the point where the pressure switch will not complete the circuit, such that the power to the system controls is lost and the system does not run. In contrast, “normally closed” switches can also be used to verify that it is safe for the furnace to come on. If the switch had failed and it was stuck open, then the furnace would not come on.
Dual, or differential, pressure switches have a normally closed and a normally open circuit. The normally closed circuit allows the furnace to safely initiate the sequence of operation resulting in a flame. Typically negative pressure is created by the expelling of the flue gases, and the normally open circuit will close. This allows the furnace to continue operating safely because the flue gases are being expelled. Most differential pressure switches have two hoses connected. The first hose is located at the vacuum side of the switch and is connected to the flue circuit (the flue circuit expels the burned gases). The second hose is located at the positive pressure side of the switch and is connected to the gas valve (the gas circuit mixes air with the gas creating the flame). Generally, there should be little or no positive pressure. Should a positive pressure exist, it is typically an indication that the primary or secondary heat exchanger is becoming plugged. As a result, pressure build up creates a positive pressure which will negate from the negative or vacuum pressure, thus causing the negative (vacuum) pressure to drop below the setting and shut the furnace down. Dual pressure switches are also used to set the gas pressure of the gas valve in high efficiency units. When the gas ignites there is a slight variance in the pressures measured by a manometer. The gas pressure is then adjusted to the manufacturer's specifications.
Faulty pressure switches may be one of the most misdiagnosed problems in today's modern furnaces. Many pressure switches have been replaced needlessly, simply because there was no proper way to test them. It is typically the technician's best guess as to whether a problem exists which necessitates replacement of the pressure switch. Thus, many service calls could have been resolved easily if the pressure switch was first able to be tested properly before being replaced. A service technician using a pressure-measuring device such as a manometer can test “static pressure” in the line to see if there is enough pressure to close the switch, but this will not reveal whether or not the pressure switch itself is working properly.
In light of this, a significant need exists in the HVAC field for the diagnosis and calibration of pressure switches. Pressure switches are “safety devices” in today's modern heating systems. These safety devices shut the heating system down should there be a problem with expelling the flue gas which contains carbon monoxide. They also insure that the system is getting enough fresh air for the correct and safe combustion of the fuel gas mixture. Since pressure switches are safety devices used on all high-efficiency heating systems used for heating residential, commercial and industrial buildings, it is extremely important that any malfunction of a pressure switch is properly diagnosed, and, if it is an adjustable pressure switch, that it is set correctly.
Prior art calibration devices also do not allow one to accurately diagnose pressure switch failure, or impending failure. Often the service technician must simply guess if a pressure switch has failed, or else guess the remaining life expectancy of a pressure switch by exchanging the pressure switch to see if the replacement switch corrected the problem. U.S. Pat. No. 7,441,439 to the present inventor McFarland, which is incorporated herein by reference in its entirety, teaches a portable pressure switch tool that can be used to create pressure or vacuum in order to test, set or adjust a pressure switch to the manufacturer's specifications while in the field. Prior to the '439 patent to McFarland, it was not possible to accurately diagnose early failure or possible failure of a pressure switch that was starting to go bad. Even worse, technicians have wasted valuable time being called back to a worksite after replacing a pressure switch, only to find out that the problem was the flue, or a blocked intake or condensate system.
While the '439 patent to McFarland teaches a device that is useful for creating pressure or vacuum in order to test, set, or adjust a pressure switch to the manufacturer's specifications while in the field, the device includes manual control valves for adjusting the vacuum. This typically requires the use of both hands in order to operate the device. Therefore, there exists a need for an HVAC service technician to be able to quickly, easily and accurately set and/or calibrate adjustable pressure switches in an HVAC system without having to operate manual control valves. It would also be advantageous to provide a hand-held calibration and diagnostic tool that can be used on pressure switches without having to use both hands to operate manual control valves. These and other features and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.